1. Technical Field
This disclosure relates to measuring lens aberration, and more particularly, to a grating pattern and a method for measuring azimuthal and radial aberrations for lenses which provide a simple and powerful way to determine the lens quality of any stepper.
2. Description of the Related Art
Optical lithography for semiconductor production systems is beginning to employ resist images with dimensions comparable to the wavelengths of light used to form the images. In an attempt to satisfy the demand for smaller images, various image enhancement schemes have been employed in the art. Some of these schemes include modifying illumination and phase shifting reticles to attempt to achieve finer images. Effective implementation of these techniques, however, requires more accurate lenses. Lens fabricators are responding by building lenses with reduced residual aberrations.
As is known in the art, all lenses have residual aberrations despite the best efforts of lens fabricators. It is difficult for lithographers to determine if the level of performance needed for a lens is in fact supplied by a particular lens without actually producing products (e.g., semiconductor products). One effective way to determine the level of performance, without producing product, is to use full wave front aberration data, such as, obtained from a Phase Measuring Interferometer (PMI) at the time of lens fabrication. Often the PMI record is not available to the lithographer, and even when available, the PMI record may not represent the lens at the time of use.
In an article by J. Kirk et al. entitled xe2x80x9cApplication of blazed gratings for determination of equivalent primary azimuthal aberrations,xe2x80x9d Proc. SPIE, vol. 3679, pg. 70 (1999) (hereinafter J. Kirk et al.), azimuthal aberrations are determined by employing a blazed grating. In J. Kirk et al., aberrations were determined from wafers exposed using a test reticle having blazed gratings with orientations from 0 to 360 degrees in increments of typical 22.5 degrees. As described in J. Kirk et al. in section 3.1, in concept, the blazed grating should be made to diffract light in only two orders. However, using the blazed grating as described in J. Kirk et al., three peak intensity regions or split peak images were seen in the image of the phase grating (see FIG. 5b of J. Kirk et al.).
It would be desirable to achieve a 2-beam illumination in the image of the phase grating as this would increase the sensitivity and the accuracy of the lens system evaluation. However, technology for fabricating a truly blazed grating is extremely difficult.
Therefore, a need exists for a simple and easy-to-manufacture grating for evaluating lenses. A further need exists for a blazed grating, which provides a true 2-beam illumination image to improve lens evaluation symmetry. A further need exists for a method for measuring azimuthal and radial aberrations for lenses to determine the lens quality for photolithographic processes.
Methods and reticles for evaluating lenses are disclosed. In one instance, a reticle which permits light to pass therethrough is provided which includes a first surface with a grating profile formed thereon. The grating profile includes a plurality of grouped stepped portions. Each group of the stepped portions includes a first step which prevents light from propagating therethrough, a second step which propagates light therethrough and a third step which propagates light therethrough at an angle 60 degrees out of phase with the light propagated through the second step.
Another embodiment for a reticle for evaluating a lens, includes a reticle, which permits light to pass therethrough. The reticle includes a first surface. A grating profile is formed on the first surface, and the grating profile includes a plurality of grouped stepped portions. Each group of the stepped portions has a first step which propagates light therethrough at a first phase angle, a second step which propagates light therethrough at an angle 90 degrees out of phase with the first phase angle and a third step which propagates light therethrough at an angle 180 degrees out of phase with the first phase angle.
Methods employing these structures are also disclosed for evaluating lens systems in accordance with the invention.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.